There are various methods of implementing an analog-to-digital (A/D) converter. The various methods include, but are not limited to, flash conversion, dual slope conversion, sigma-delta conversion and successive approximation conversion. Each of the various analog-to-digital conversion methods may be implemented using a variety of circuitry. The circuitry may include, but is not limited to, a resistor ladder, a plurality of appropriately sized capacitors, and a plurality of constant current sources. The various combinations of digital conversion methods and circuitry for implementing analog-to-digital converters each have advantages and disadvantages. The advantages and disadvantages are typically expressed in terms of speed of conversion, accuracy of conversion, power consumption during operation, and cost of implementation.
Examples of analog-to-digital converters that utilize a successive approximation method of conversion are discussed within U.S. Pat. No. 4,358,752 entitled "Analog-To-Digital Converter", by Tamada et al.. FIG. 1 of U.S. Pat. No. 4,358,752, is a block diagram of an analog-to-digital converter utilizing a comparator, a successive approximation register (SAR), and a digital-to-analog converter. Analog-to-digital converters which utilize current sources share a common aspect in that current available from current sources within the digital-to-analog portion of the analog-to-digital converter is not utilized as efficiently as possible. That is, during the conversion operation the current sources are alternately connected, via a switch, to either an input terminal of a comparator or a terminal of a power supply. During the time period in which the current source is connected to the terminal of the power supply, power is inefficiently consumed by shunting current from the power supply via the switch through the current source. Other examples of analog-to-digital converters that have this feature are illustrated in U.S. Pat. No. 4,764,753 entitled "Analog To Digital Converter", by Yukawa, and U.S. Pat. No. 4,590,456 entitled "Low Leakage CMOS D/A Converter", by Burton et al.. Other disadvantages with known A/D converters utilizing current steering include a lack of sufficient noise immunity. Typically, in order to achieve better noise immunity, an increase in power consumption is required.